Just as Mario is the leading mascot of Nintendo, Sonic managed to secure his spot as Sega's leading star. He had appeared in quite a few games on the Mega Drive. Sonic & Knuckles does something quite different from the rest of the series, and different from the rest of the market in general. Aside from giving us an entirely new character this game is a rare piece of hardware that can "lock-on" to other game cartridges. After some time playing I can conclude that this is no mere gimmick, and that the rest of the product hasn't been forgotten.

Well, for all its differences there is still a fairly distinct sense of familiarity to the game for fans of the series, as S&K carries over many of the features presented in the series, and especially those of Sonic 3. This is not a bad thing, as these features melded together for an excellent experience.

Speed is a major focus in any Sonic game, and S&K does this perfectly. Both playable characters can run at high speeds, although needing a little time to either make a run-up or perform a spin dash. At full pelt the landscape flies past almost at a blur, and this is really the defining aspect of the series. Dodging past hazards is one thing, but doing it at sonic speed is an entirely different experience. Put simply, if you're constantly moving at a slow pace you're not playing the game right. Stop being a wuss and start running.

Obviously, simply being able to run fast wouldn't be much of an accomplishment, which is where the level design comes in. Let's face it, S&K's level design is so awesome it mocks many other side-scrollers of its era with its superiority. Every area of the game is built to accommodate this intense speed, as conveniently placed springs, loops, ramps, twists and all sorts of other terrain features that are either designed purely to be taken at speed or designed to boost you up to high speeds.

On the way there will be tons of hazards to watch out for, including the badnik enemies, spike pits, flamethrowers, crusher walls, bottomless chasms and all manner of dangers. This is the whole point of the game. First you get up to speed, and then you react to everything that comes. There's nothing quite like tearing along a slope, zooming around a loop, launching off a ramp only to then deftly slide under an enemy and then leap over spikes to land on a faraway platform. The developers were also clever enough to not overload the game with hazards, and the thrill of high speeds is always there to entice the player to rush off at a moment's notice.

Yet the level design doesn't stop with encouraging speed runs. Each act is simply massive, presenting a level of complexity all games of the genre should aspire to. Alternate pathways, hidden openings, secret chambers. Each level is so filled to the brim with all these that you could play a level three times and take almost completely different routes each time and still miss a few goodies hidden in the furthest recesses of the stage. Exploring is so tempting to see if you'll find a new shield or some more rings, although the pressure of the timer will prevent you from wasting too much time in one act.

There are also so many special features - many of which are unique to the zone you are in - that will affect the gameplay drastically. You have grab handles that allow you to rappel down walls, spinning cylinders that allow you to reach upper areas by running at full speed at them or one zone will sometimes have you running along the ceiling. One act inparticular has a really interesting trait of the lights slowly going out and ghosts appearing when they do, but light switches at set points keep them at bay, almost requiring fast movement to make sure you reach the next switch in time.

In addition to the main levels there are a few bonus areas that play out drastically different. Pass one of the starposts (which act as checkpoints in case you die) in any stage with 50 rings or more and a ring of stars will appear, which lead to one of two bonus areas. One is a kind of slot machine, where jumping into the central slot will cause the reels to spin and can give or remove rings from you based on the result. Movement is purposely awkward to try and prevent you from snagging too many goes at it, and you exit when you land on an exit circle. The second one is an odd vertical stage where you latch onto oversized orbs to spin around them. Clever timing of releasing the grip of an orb is needed to catapult upwards. Along the way you can grab items like coins and shields, and this area ends either when you reach the top or the bean rising from the bottom catches you. These bonus areas are quite fun and great for earning extra items.

Another type of bonus stage exists that forms the game's main sidequest. Throughout almost every level there are large rings hidden around the place that respond when you have 50 rings or more. Jumping into one transports you to a strange 3D environment. Here your character runs automatically once started and you have to turn them at the connecting points of the square edges. In these areas are various orbs, and the goal is to collect all the blue orbs while avoiding the red orbs. There's a certain trick to grabbing the blue orbs by grabbing all the edge orbs from square or rectangle groups. It sounds odd but once you play you'll understand and it is another great bonus area. Complete one successfully to get one of the seven chaos emeralds. These emeralds are not only for altering the end sequence and opening an extra level for Sonic but they also offer another gameplay element once all the seven are collected. At that point you can collect fifty rings and then perform a midair jump to transform into the super version of your character. Like this you have invincibility to anything except bottomless pits and crushers, plus enhanced speed, but at the cost of your rings depleting as time passes. The one problem with this is that since the activation method is the same as Knuckles gliding option then it means sometimes Knuckles can't help but activate just so he can progress. Midly annoying but nothing major.

There are a few items for the duo to grab on their journey. Rings are the common pickup, offering points and an extra life for collecting 100. These rings also provide the last line of defence. Get hit by an enemy or hazard and watch as your current ring supply gets scattered. It's possible to pick up the rings as they bounce away, helping to keep you alive if you're fast enough. Extra lives can also be found in the form of TV sets featuring your character's face. The special shields have also carried over from Sonic 3. The fire shield (invincible from fire plus a special air dash for Sonic) is quite useful and so is the lightning shield (attract nearby rings to you and a midair jump for Sonic). The water shield is much less useful. It works as an extra layer of protection to prevent your rings from getting scattered, but its special traits aren't useful. I mean, having Sonic suddenly crash downwards has no real benefit, and the ability to breathe underwater is pointless since S&K, for some reason, doesn't actually have any underwater sections at all.

The levels, and gameplay in general, are affected by the character you pick. Sonic will be familiar to fans, but the newcomer Knuckles is the one offering the fresh experience even fans won't have done before. Both characters possess a lot of similar moves, such as running fast, the spin dash (the ability to curl and rev up on the spot for a quick burst of speed from a stationary spot) and the jumping spin attack move (useful for crashing into baddies to defeat them).

Sonic seems to have a little more speed and jumping height than Knuckles, allowing him to reach a few areas that would barely be out of reach for the red bruiser. However, Knuckles comes with his own set of extra moves. Those oversized fists are capable of smashing through some walls simply by running into them, but the real treat are gliding and climbing. Pressing and holding jump in the air will allow Knuckles to glide across the screen, while gliding into a solid wall will cause him to latch on and then climb up and down the surface. The special traits of both characters, as well as some character-specific events, expands on the levels even further by having certain areas only one of them can reach.

The game is massive. Not only is each level very big but there are quite a few levels spread through the game. The last part of the game even completely changes depending on who you play as. That said, this does highlight the game's one major flaw - there's no save system. This is very disappointing, as surely late generation 16 bit titles should not be coming through without any method of saving your progress. Sonic 3 actually had one, so why S&K doesn't is beyond me.

So let's take a look at that special feature. The top of the cartridge opens to reveal another connector, allowing you to plug a game cart on there. Technically you can plug any game there, but the major special features only work on Sonic 2 or Sonic 3. Both games allow you to play through them as Knuckles (with Sonic 3 even connecting the two adventures, therefore allowing the Tails fans to take their favourite fox through the S&K stages if they want). S&K was already an awesome package on its own merits, but this setup is amazing, as it really adds new life to the older games that you might have already played to death.

S&K's gameplay is therefore fantastic, but the team spared no effort on the presentation as well. Each zone of the floating island and connecting areas are rendering beautifully. From the grassy hillsides to volcanic rock formations to the sparkling mystic palace. Every area is filled with a tremendous amount of detail in both the foreground and background, really bringing each zone to life and there is such variation in each area - sometimes even between acts of the same zone - that you could never get bored of its looks.

The character sprites are marvellous to look at as well. As well as being colourful and well designed, each sprite also has fluid and often charming animations. There are even the cool extra stuff, like leaving either character alone and watch them get impatient or the celebration part at the end of an act.

The music is as great as fans expect. The music really does suit the fantasy mood set forth by the game's content and many of the tracks are quite catchy. The quality is superb too. Sound effects are very well done and match up well to the actions happening in the game. This is how 16 but gaming audio should be done.

Sonic & Knuckles is no gimmick. The Lock-On feature is excellent, but even as a stand alone title this is easily one of the best titles to come out of Sega's 16 bit days. Even now this game can still appeal greatly to many gamers, and that is a testament to its quality.